Imager Makes 3-D Pictures From Half a Mile Away

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Pete Townshend wrote “I can see for miles and miles.” A new twist on laser scanning may make that almost true — and in 3D.

Making 3-D images is already done with lasers. A projector fires a laser beam at something and a detector measures how long it takes the beam to come back. That tells a computer how far away the object is. Repeated thousands of times for each pixel of the image, the computer can then generate a 3-D picture.

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The problem with these systems is that they don’t work with anything that’s far away. Visible light lasers get interrupted by sunlight. Also, these lasers get absorbed easily by a range of materials, like clothing, and so little reflected light bounces back to create an image.

Aongus McCarthy, a research associate at Heriot-Watt University in Edinburgh, Scotland, and his colleagues in the laboratory of Gerald Buller, decided to solve that problem. They used longer-wavelength lasers, a detector that can pick up single photons and a sophisticated image processing program. With it they can get 3-D images of objects as far away as 1,093 yards, or about six tenths of a mile — not quite what The Who song says, but it’s farther than any other system so far.

It works because of the detector and the laser the scientists chose. Instead of using a laser that uses visible light, Buller’s team employed an infrared laser. These lasers travel farther because they aren’t disrupted as much by sunlight. On top of that, infrared light is reflected by a wider range of objects — it won’t get absorbed by clothing. It’s also a lot safer for people’s eyes.

Then there’s the detector that picks up the reflected light. The Watt University group designed one that is more sensitive, and picks up even individual photons — it can see much fainter reflections than conventional designs.

Their imager would be useful for military personnel, for example, who want to know what a distance object is, but that is too far away to see. But 3-D images aren’t just for surveillance. Getting a good 3-D picture of a mountainside or snowy peak could tell park rangers whether landslides or avalanches are likely, for instance.

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Despite it’s ability to see so far, the imager can’t see faces — one of the things that doesn’t reflect infrared light well is human skin, though that changes if you sweat. And the algorithm that builds the pictures is still on the slow side — it takes several seconds to make a picture. The team also wants to improve the range, and it’s possible, they think, to push it to 6 miles.

But the day might come when the telephoto lens is obsolete, and what would it have meant for the guy in this Hitchcock classic?

The details of the new imager appear in the journal Optics Express.

Photo: Comparison between the results obtained from scans, of a mannequin and a person, at a range of 325 meters.

Credit: Heriot-Watt University

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